Circuits of this type are used in field devices, for example. There, they may be used in a superordinate unit, e.g., a transmitter, that supplies an electric load, e.g., a sensor, with power via an interface that can be connected thereto, and receives desired signals transmitted from said electric load. Information about this, e.g., characteristic values of the sensor and/or measurement values determined by the sensor, may be transferred from the electric load to the superordinate unit.
For this, FIG. 1 shows a block diagram of a circuit 1 known from the prior art for supplying power to an electric load 5 that can be connected to the circuit 1 via an interface 3, and for receiving desired signals transmitted from the electric load by means of an amplitude modulation, performed by the electric load, of a carrier signal transmitted from the circuit 1 via the interface 3, said carrier signal serving to supply power to the electric load 5.
For a better understanding of the circuit 1, here, the circuit 1 is depicted together with the interface 3 connected thereto and the electric load 5 connected to the interface.
The interface 3 here is designed as an inductive interface 3 that enables a wireless transfer of power and desired signals. This comprises a circuit-side inductance L1, connected to the circuit 1, that forms a transmitter together with an electric load-side inductance L2 connected to the electric load 5.
For example, this interface 3 may be integrated into a plug connector via which a superordinate unit comprising the circuit 1 may simultaneously be connected to the electric load 5 mechanically, and also electrically. Plug connectors equipped with an inductive interface, as they are distributed by the applicant under the designation MEMOSENS, are an example of this. These plug connectors comprise two components that can be mechanically connected to one another, in which is respectively enclosed an inductance L1, L2 that forms an inductive transmitter, together with the inductance L2, L1 arranged in the respective other component connected thereto.
The circuit 1 comprises a carrier signal generator having a direct voltage source UDC and a DC-AC converter 7 downstream of the direct voltage source UDC, at the output of which DC-AC converter 7 is provided a carrier signal having a carrier signal frequency set by the DC-AC converter 7.
The electric load 5 comprises an element 9 to be supplied with power via the interface 3 for example, the sensor S connected via a rectifier 11 to the two terminals of the electric load-side inductance L1.
Moreover, the electric load 3 comprises a modulator 13 for modulating the amplitude of the carrier signal transmitted via the interface 3. This comprises a load 15 connected parallel to the element 9 to be supplied, which load 15 is connected via a controllable switch (not designated in detail here) upstream of said load 15, at times predetermined by the digital desired signal to be transmitted from the electric load 5.
The connection of the load 15 produces a discontinuous voltage drop, the voltage dropping across the circuit-side inductance L1. Accordingly, a demodulator 17 connected in parallel to the circuit-side inductance L1 is used to extract the desired signal, at the inputs of which demodulator 17 is the voltage dropping across said circuit-side inductance L1. The demodulator 17 comprises a rectifier 19, a bandpass filter 21 downstream of the rectifier, and a comparator 23 with an adjustable comparator threshold downstream of the bandpass filter 21, at the output of which is provided an information signal reflecting the received desired signal.
However, this form of desired signal transfer requires that the voltage drop produced by the connection of the load 15 not fall below a minimum value that can be reliably detected via the demodulator 17 connected in parallel to the load 15.
The cause of the voltage drop is the electric load current flowing across the load upon connection of the load in parallel with the current flowing through element 9 to be supplied. With regard to this current flow, the DC-AC converter 7, the interface 3, and the load 15 form impedances connected in series. So that the electric load current flowing across the load 15 upon connection of said load 15 produces an optimally large voltage drop of the voltage dropping across the circuit-side inductance L1, the DC-AC converter 7 must have an optimally high impedance. For this reason, amplifiers with high impedance, e.g., the Class E amplifier shown in FIG. 1, are used as DC-AC converters 7 in the prior art. These amplifiers typically comprise resonators clocked via a switching step, upstream of which are connected in the series arm an ohmic resistor R and a choke coil LD connected thereto in series. Via a correspondingly high impedance of the series circuit of resistor R and choke coil LD, it is thereby ensured that a sufficiently large voltage drop is also produced via the connection of the load 15, when the element 9 of the electric load 5 to be supplied with power consumes a comparably large amount of power.
However, it is disadvantageous that a correspondingly high power loss is realized due to the resistive portion of this impedance in the amplifier. This power loss is not provided for supplying the electric load 5, and accordingly degrades the efficiency of the circuit 1.
An additional disadvantage is that choke coils LD are comparably large components that not only take up a large amount of space, but also are mechanically sensitive. The latter, especially, represents a problem when the circuit 1 is to be used in a housing that is subsequently filled with a potting compound. For example, such a potting compound is used in order to avoid a penetration of moisture into the housing. Damage to the choke coil LD installed on the circuit board may thereby already occur upon potting of the internal space. Moreover, a later thermal expansion of the cured potting compound may also lead to damage to the choke coil LD and/or to it making contact.